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Let $H$ be a separable Hilbert space and let $\operatorname{U}(H)$ be the group of unitary transformations of $H$. It is well known that the weak, strong and compact-open topologies on $\operatorname{U}(H)$ all agree (Espinoza and Uribe - Topological properties of the unitary group).

In particular, they all induce the standard topology on the center $\operatorname{U}(1) \subset \operatorname{U}(H)$, and the quotient $P{\operatorname{U}(H)} = \operatorname{U}(H)/{\operatorname{U}(1)}$ is a topological group.

Now, $P{\operatorname{U}(H)}$ is the automorphism group of the $C^*$-algebra $A = \operatorname{B}(H)$, and for generic $C^*$-algebras, one usually considers the "point-norm-topology" on the automorphism group $\operatorname{Aut}(A)$, which is just the restriction of the strong topology on space of all bounded operators on $A$.

On the other hand, since $\operatorname{B}(H)$ is a von Neumann algebra, the more natural topology may be Haagerup's u-topology (see Haagerup - The standard form of von Neumann algebras). By Corollary 3.8 in Haagerup's paper, for $\operatorname{B}(H)$ this coincides with the p-topology, which is the restriction of the locally convex topology on the set of all $\sigma$-weakly bounded transformations of $A$ given by the collection of seminorms $$\varphi \mapsto \lvert\xi(\varphi(a))\rvert, ~~\xi \in A_* ,a \in A, \varphi \in \operatorname{Aut}(A).$$ Here $A_*$ denotes the predual, which in our case is the space of trace-class operators.

Q: Do these two topologies on $P{\operatorname{U}(H)}$ also coincide, and are they equal to the quotient topology of the weak/strong topology?

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  • $\begingroup$ What is the locally convex topology on the space of all bounded operators? $\endgroup$
    – user473423
    Commented Sep 2, 2023 at 3:57
  • $\begingroup$ Isn't it true that for a separable Hilbert space 𝐻 the u-topology on U(𝐻) and the weak topology coincide? $\endgroup$
    – user473423
    Commented Sep 2, 2023 at 4:53
  • $\begingroup$ @Echo: Reworded the definition of the point-norm topology appropriately. Regarding, your second comment: This is precisely the question (or one of them). If you have an answer or a reference, it would be great if you could add that. $\endgroup$ Commented Sep 3, 2023 at 11:44
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    $\begingroup$ The $u$-topology is the "right" topology for $\mathrm{Aut}(M)$ for a von Neumann algebra $M$ (which is $B(H)$ in this case) and so it gives us the right topology on $P\mathrm{U}(H)$, which is the quotient SOT. The point-norm topology seems to give rise to a pathological topology, which is definitely not the quotient SOT nor norm. $\endgroup$ Commented Sep 4, 2023 at 2:32
  • $\begingroup$ @NarutakaOZAWA: Could you elaborate on why the point-norm topology does not coincide with the other two topologies, and what you mean by "pathological"? $\endgroup$ Commented Sep 5, 2023 at 12:14

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